Tea Party favorite Chris McDaniel and six-term Sen. Thad Cochran dueled inconclusively at close quarters in Mississippi's primary election Tuesday night, an epic struggle in a party deeply divided along ideological lines. GOP governors in South Dakota, Alabama and Iowa all coasted to renomination.

Senate hopeful Joni Ernst, a state senator, overwhelmed a fistful of Republican rivals in Iowa after uniting rival wings of the party and will challenge Rep. Bruce Braley this fall for a Senate seat long in Democratic hands.

In a third Senate race on the busiest night of the primary season, former Gov. Mike Rounds won the Republican nomination in South Dakota - and instantly became the favorite to pick up a seat for the GOP in its drive to capture the six the party needs to capture a majority this fall.

The marquee contest of the night was in Mississippi, where Cochran, 76, and the 41-year-old McDaniel remained locked in a close, uncallable race as the vote count mounted. Returns from 98 percent of the state's precincts showed the challenger narrowly ahead in a three-way race, but just below the 50 percent threshold needed to avoid a June 24 runoff.

"People of this country were somehow awakened and we've been asleep for far too long," McDaniel told supporters as the results came in.

Rep. Gregg Harper, who had campaigned with Cochran, told the senator's supporters, "It's looking like a runoff."

Officials said the vote tally did not include provisional ballots, at least some of them cast as a result of the state's new voter ID law. Those voters have five days to furnish proof of residence. An official canvass could take longer, until June 13.

The Senate contest between Cochran and McDaniel in Mississippi drew top billing, a costly and heated race between a pillar of the GOP establishment who has helped funnel millions of dollars to his state and a younger state lawmaker who drew backing from Tea Party groups and former Republican vice presidential candidate Sarah Palin. The campaign took a turn toward the sensational when four men, all McDaniel supporters, were arrested and charged with surreptitiously taking photographs of the senator's 72-year-old wife, who suffers from dementia and has long lived in a nursing home.

The race was arguably the year's last good chance for the Tea Party wing of the party to topple an establishment favorite in a Senate primary, following losses in Texas, North Carolina, Georgia and Kentucky.

In other Senate races, appointed Democratic Sen. John Walsh and Republican Rep. Steve Daines in Montana each overpowered primary rivals en route to a likely race in the fall that the GOP is expected to target as an opportunity to gain a seat.

Republicans eyed another fall pickup opportunity in South Dakota, where Democratic Sen. Tim Johnson is retiring and Rounds easily eclipsed his rivals for the GOP nomination. Rick Weiland, making his third try for a seat in Congress, was unopposed by other Democrats.

Sen. Cory Booker, D-N.J., had no competition for renomination, and Jeff Bell won the GOP spot on the November ballot.